1. Field of the Invention
The present invention relates in general to automatic transmissions of step type (viz., non-continuously variable type), and more particularly to the automatic transmissions of a type that is equipped with means for sensing an undesired interlock of an input or output shaft of the transmission.
2. Description of the Related Art
Automatic transmissions of the above-mentioned step type are constructed to assume desired speeds by selectively connecting and disconnecting a plurality of frictional elements with the aid of hydraulic actuators. These actuators comprise valves with spools. If, under cruising of an associated motor vehicle, some valve spool of the actuators is subjected to a stick condition or some engaged frictional element is subjected to such stick condition for some reason, undesired interlock takes place in the transmission, which causes a sudden deceleration of the vehicle against will of the driver. As is know, the interlock is a locked condition of input or output shaft of the transmission which is induced when intended engagement of one frictional element and unintended engagement of the other frictional element take place at substantially same time. Of course, such sudden deceleration of the vehicle caused by the interlock makes the driver as well as other passengers of the vehicle uncomfortable.
In order to solve and eliminate such undesired phenomenon, various measures have been hitherto proposed and put into practical use, one of which is disclosed in Japanese Laid-open Patent Application (Tokkaisho) 61-165056. In the measure of this publication, a pressure sensor is arranged in a hydraulic circuit of each frictional element, and if the pressure sensor senses a certain degree of hydraulic pressure in the hydraulic circuit under a condition wherein no instruction signal for such hydraulic pressure is issued, judgment is so made that an interlock has taken place in the transmission. Furthermore, if, during a ratio change operation, a variation in acceleration of the vehicle between before and after the ratio change exceeds a given value, it is judged that an interlock may have taken place.